New perspectives for perfusion imaging in echocardiography

The rapid injection into the coronary circulation of solutions containing microbubbles produces an ultrasonic contrast effect in the myocardium. The time-intensity curves generated by sequential videodensitometric analysis of contrast intensity, which is corrected for myocardial background intensity, resembles the curves used in indicator dilution techniques. Studies done in vitro have demonstrated a direct relation between contrast intensity and amount of microbubbles. In animal studies, measurements derived from these curves correlated with changes in myocardial blood flow assessed by microspheres but with varying and controversial results. Differences between investigators in technique of injection and size of microbubbles injected are among the list of factors that will alter the time-intensity curve and explain some of the differences in results between investigators. Several factors limit the application of myocardial contrast echocardiography (MCE) to the quantitation of myocardial blood flow. Nevertheless, the results from multiple investigations suggest that the technique is sensitive to changes in perfusion and may be applicable to the evaluation of regional coronary reserve and assessment of the results of revascularization procedures. Peak intensity and area under the time-intensity curve have provided consistent results between investigators; therefore, these measurements have been used to assess regional coronary reserve in experimental studies and patients with coronary artery disease. Although the results are encouraging, they cannot distinguish well between normal and mild impairments in coronary reserve and are subject to larger reproducibility errors. A potential advantage of MCE is that perfusion can be assessed in a segment independent of changes in neighboring segments. In addition, the technique seems sensitive to changes in perfusion between the subendocardium and subepicardium and to perfusion by collateral flow in segments supplied by an occluded coronary artery. These features of the technique are important because presently no methods exist for this type of evaluation in vivo. However, the future of MCE depends on the development of better contrast agents, establishment of accurate and reproducible measurement algorithms, and improvements in ultrasound instrumentation, resulting in a more accurate measurement of the intensity of reflected ultrasound.